Method And System For Germanium-On-Silicon Photodetectors Without Germanium Layer Contacts

ABSTRACT

Methods and systems for germanium-on-silicon photodetectors without germanium layer contacts are disclosed and may include, in a semiconductor die having a photodetector, where the photodetector includes an n-type silicon layer, a germanium layer, a p-type silicon layer, and a metal contact on each of the n-type silicon layer and the p-type silicon layer: receiving an optical signal, absorbing the optical signal in the germanium layer, generating an electrical signal from the absorbed optical signal, and communicating the electrical signal out of the photodetector via the n-type silicon layer and the p-type silicon layer. The photodetector may include a horizontal or vertical junction double heterostructure where the germanium layer is above the n-type and p-type silicon layers. An intrinsically-doped silicon layer may be below the germanium layer between the n-type silicon layer and the p-type silicon layer. A top portion of the germanium layer may be p-doped.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application is a continuation of U.S. patent application Ser. No.14/926,916 filed on Oct. 29, 2015, now U.S. Pat. No. 10,546,963, whichclaims priority to and the benefit of U.S. Provisional Application62/086,137 filed on Dec. 1, 2014, each of which is hereby incorporatedherein by reference in its entirety.

FIELD

Certain embodiments of the disclosure relate to semiconductor photonics.More specifically, certain embodiments of the disclosure relate to amethod and system for germanium-on-silicon photodetectors withoutgermanium layer contacts.

BACKGROUND

As data networks scale to meet ever-increasing bandwidth requirements,the shortcomings of copper data channels are becoming apparent. Signalattenuation and crosstalk due to radiated electromagnetic energy are themain impediments encountered by designers of such systems. They can bemitigated to some extent with equalization, coding, and shielding, butthese techniques require considerable power, complexity, and cable bulkpenalties while offering only modest improvements in reach and verylimited scalability. Free of such channel limitations, opticalcommunication has been recognized as the successor to copper links.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with the present disclosure as set forth inthe remainder of the present application with reference to the drawings.

BRIEF SUMMARY

A system and/or method for germanium-on-silicon photodetectors withoutgermanium layer contacts, substantially as shown in and/or described inconnection with at least one of the figures, as set forth morecompletely in the claims.

Various advantages, aspects and novel features of the presentdisclosure, as well as details of an illustrated embodiment thereof,will be more fully understood from the following description anddrawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a block diagram of a photonically-enabled integrated circuitwith germanium detectors without germanium layer contacts, in accordancewith an example embodiment of the disclosure.

FIG. 1B is a diagram illustrating an exemplary photonically-enabledintegrated circuit, in accordance with an example embodiment of thedisclosure.

FIG. 1C is a diagram illustrating a photonically-enabled integratedcircuit coupled to an optical fiber cable, in accordance with an exampleembodiment of the disclosure.

FIG. 2 illustrates a germanium photodetector with contacts on germaniumlayers, in accordance with an example embodiment of the disclosure.

FIG. 3A illustrates a germanium photodetector without contacts ongermanium, in accordance with an example embodiment of the disclosure.

FIG. 3B illustrates the band diagram for the photodetector structureshown in FIG. 3A.

FIG. 4 illustrates a cross-section of a horizontal doubleheterostructure, in accordance with an example embodiment of thedisclosure.

FIG. 5 illustrates a vertical double heterojunction germaniumphotodetector, in accordance with an example embodiment of thedisclosure.

FIG. 6 illustrates a surface-illuminated horizontal doubleheterostructure germanium photodetector, in accordance with an exampleembodiment of the disclosure.

FIG. 7 illustrates a vertical junction surface-illuminated photodiode,in accordance with an example embodiment of the disclosure.

FIG. 8 illustrates a test structure for assessing n-type tunnelingcontacts, in accordance with an example embodiment of the disclosure.

FIG. 9 illustrates a test structure for assessing p-type tunnelingcontacts, in accordance with an example embodiment of the disclosure.

FIG. 10 illustrates 4-probe heterostructure test structures, inaccordance with an example embodiment of the disclosure.

DETAILED DESCRIPTION

Certain aspects of the disclosure may be found in a method and systemfor germanium-on-silicon photodetectors without germanium layercontacts. Exemplary aspects of the disclosure may comprise, in asemiconductor die having a photodetector comprising an n-type siliconlayer, a germanium layer, a p-type silicon layer, and a metal contact oneach of the n-type silicon layer and the p-type silicon layer: receivingan optical signal, absorbing the optical signal in the germanium layer,generating an electrical signal from the absorbed optical signal, andcommunicating the electrical signal out of the photodetector via then-type silicon layer and the p-type silicon layer. The photodetector maycomprise a horizontal junction double heterostructure where thegermanium layer is above the n-type silicon layer and the p-type siliconlayer. An intrinsically-doped silicon layer may be below the germaniumlayer between the n-type silicon layer and the p-type silicon layer. Aportion of the germanium layer nearest the p-doped silicon layer may bep-doped. The photodetector may comprise a vertical junction doubleheterostructure where the germanium layer is above a lower-doped n-typesilicon layer. The n-type silicon layer and the p-type silicon layersmay be on opposite sides of the lower-doped silicon layer below thegermanium layer where the p-type silicon layer and the lower-dopedn-type silicon layer are in contact with the germanium layer while then-type silicon layer is not. A top portion of the germanium layer may bedoped p-type. The photodetector may comprise a surface-illuminateddouble heterostructure photodetector. The n-type silicon layer and thep-type silicon layer in the surface-illuminated double heterostructurephotodetector may comprise interdigitated fingers. The semiconductor diemay be a silicon complementary-metal oxide semiconductor (CMOS) die.

FIG. 1A is a block diagram of a photonically-enabled integrated circuitwith germanium detectors without germanium layer contacts, in accordancewith an example embodiment of the disclosure. Referring to FIG. 1A,there are shown optoelectronic devices on a photonically-enabledintegrated circuit 130 comprising optical modulators 105A-105D,photodiodes 111A-111D, monitor photodiodes 113A-113H, and opticaldevices comprising couplers 103A-103K, optical terminations 115A-115D,and grating couplers 117A-117H. There are also shown electrical devicesand circuits comprising amplifiers 107A-107D, analog and digital controlcircuits 109, and control sections 112A-112D. The amplifiers 107A-107Dmay comprise transimpedance and limiting amplifiers (TIA/LAs), forexample.

In an example scenario, the photonically-enabled integrated circuit 130comprises a CMOS photonics die with a laser assembly 101 coupled to thetop surface of the IC 130. The laser assembly 101 may comprise one ormore semiconductor lasers with isolators, lenses, and/or rotators withinfor directing one or more CW optical signals to the coupler 103A. Thephotonically enabled integrated circuit 130 may comprise a single chip,or may be integrated on a plurality of die, such as with one or moreelectronics die and one or more photonics die.

Optical signals are communicated between optical and optoelectronicdevices via optical waveguides 110 fabricated in thephotonically-enabled integrated circuit 130. Single-mode or multi-modewaveguides may be used in photonic integrated circuits. Single-modeoperation enables direct connection to optical signal processing andnetworking elements. The term “single-mode” may be used for waveguidesthat support a single mode for each of the two polarizations,transverse-electric (TE) and transverse-magnetic (TM), or for waveguidesthat are truly single mode and only support one mode whose polarizationis TE, which comprises an electric field parallel to the substratesupporting the waveguides. Two typical waveguide cross-sections that areutilized comprise strip waveguides and rib waveguides. Strip waveguidestypically comprise a rectangular cross-section, whereas rib waveguidescomprise a rib section on top of a waveguide slab. Of course, otherwaveguide cross section types are also contemplated and within the scopeof the disclosure.

The optical modulators 105A-105D comprise Mach-Zehnder or ringmodulators, for example, and enable the modulation of thecontinuous-wave (CW) laser input signal. The optical modulators105A-105D may comprise high-speed and low-speed phase modulationsections and are controlled by the control sections 112A-112D. Thehigh-speed phase modulation section of the optical modulators 105A-105Dmay modulate a CW light source signal with a data signal. The low-speedphase modulation section of the optical modulators 105A-105D maycompensate for slowly varying phase factors such as those induced bymismatch between the waveguides, waveguide temperature, or waveguidestress and is referred to as the passive phase, or the passive biasingof the MZI.

The outputs of the optical modulators 105A-105D may be optically coupledvia the waveguides 110 to the grating couplers 117E-117H. The couplers103D-103K may comprise four-port optical couplers, for example, and maybe utilized to sample or split the optical signals generated by theoptical modulators 105A-105D, with the sampled signals being measured bythe monitor photodiodes 113A-113H. The unused branches of thedirectional couplers 103D-103K may be terminated by optical terminations115A-115D to avoid back reflections of unwanted signals.

The grating couplers 117A-117H comprise optical gratings that enablecoupling of light into and out of the photonically-enabled integratedcircuit 130. The grating couplers 117A-117D may be utilized to couplelight received from optical fibers into the photonically-enabledintegrated circuit 130, and the grating couplers 117E-117H may beutilized to couple light from the photonically-enabled integratedcircuit 130 into optical fibers. The grating couplers 117A-117H maycomprise single polarization grating couplers (SPGC) and/or polarizationsplitting grating couplers (PSGC). In instances where a PSGC isutilized, two input, or output, waveguides may be utilized.

The optical fibers may be epoxied, for example, to the CMOS chip, andmay be aligned at an angle from normal to the surface of thephotonically-enabled integrated circuit 130 to optimize couplingefficiency. In an example embodiment, the optical fibers may comprisesingle-mode fiber (SMF) and/or polarization-maintaining fiber (PMF).

In another exemplary embodiment illustrated in FIG. 1B, optical signalsmay be communicated directly into the surface of thephotonically-enabled integrated circuit 130 without optical fibers bydirecting a light source on an optical coupling device in the chip, suchas the light source interface 135 and/or the optical fiber interface139. This may be accomplished with directed laser sources and/or opticalsources on another chip flip-chip bonded to the photonically-enabledintegrated circuit 130.

The photodiodes 111A-111D may convert optical signals received from thegrating couplers 117A-117D into electrical signals that are communicatedto the amplifiers 107A-107D for processing. In another embodiment of thedisclosure, the photodiodes 111A-111D may comprise high-speedheterojunction phototransistors, for example, and may comprise germanium(Ge) in the collector and base regions for absorption in the 1.3-1.6 μmoptical wavelength range, and may be integrated on a CMOSsilicon-on-insulator (SOI) wafer.

In conventional integrated electronic devices, metal plugs are used toestablish a contact to the device electrodes. For example, tungstenplugs are commonly used to establish contact to the drain, source, andgate terminals of MOS transistors, or to the anode and cathode of diodesand photodiodes. This type of contact provides a compact, reliable andlow-contact-resistance way of supplying the voltage and carrying thecurrents in and out of the devices. However, especially wheregermanium-on-Si integrated photo-detectors are concerned, the followingissues arise when contacts are formed on a germanium active region: 1)the presence of contacts on the germanium implies that metal plugs areplaced close to the optical mode in the active region of thephoto-detector, where absorption occurs, thereby causing scatteringloss, thus impairing responsivity; 2) efficient electrical contacts togermanium require high doping in the contact region, but in aphotodetector, the low electric field in the highly doped regionsresults in poor photo-carrier collection from those areas, therebydegrading the quantum efficiency of the device; and 3) the process forcontact formation requires the exposure of the Ge film to aggressivecleaning solutions. Germanium is corroded by water since its oxide iswater soluble, so these cleaning solutions may cause significant damageto the device.

An alternative approach to the use of metal plugs directly contactingthe Ge film, which is compatible with existing process technology andprovides a complete solution to the issues mentioned above, is describedin this disclosure. In an example scenario, the siliconabutting/surrounding the germanium films assumes an active electricalrole by providing the means through which voltage and currents areapplied and flow as well as the chemical potential difference causingthe rise of the junction built-in voltage. This may be accomplished byrealizing a double heterostructure (DH) device in which the currentflowing into the germanium device is injected/extracted through highlydoped p- and n-type Si/Ge heterojunctions, where the doping is solely inthe silicon layers.

The analog and digital control circuits 109 may control gain levels orother parameters in the operation of the amplifiers 107A-107D, which maythen communicate electrical signals off the photonically-enabledintegrated circuit 130. The control sections 112A-112D compriseelectronic circuitry that enable modulation of the CW laser signalreceived from the splitters 103A-103C. The optical modulators 105A-105Dmay require high-speed electrical signals to modulate the refractiveindex in respective branches of a Mach-Zehnder interferometer (MZI), forexample. In an example embodiment, the control sections 112A-112D mayinclude sink and/or source driver electronics that may enable abidirectional link utilizing a single laser.

In operation, the photonically-enabled integrated circuit 130 may beoperable to transmit and/or receive and process optical signals. Opticalsignals may be received from optical fibers by the grating couplers117A-117D and converted to electrical signals by the photodetectors111A-111D. The electrical signals may be amplified by transimpedanceamplifiers in the amplifiers 107A-107D, for example, and subsequentlycommunicated to other electronic circuitry, not shown, in thephotonically-enabled integrated circuit 130.

Integrated photonics platforms allow the full functionality of anoptical transceiver to be integrated on a single chip. An opticaltransceiver chip contains optoelectronic circuits that create andprocess the optical/electrical signals on the transmitter (Tx) and thereceiver (Rx) sides, as well as optical interfaces that couple theoptical signals to and from a fiber. The signal processing functionalitymay include modulating the optical carrier, detecting the opticalsignal, splitting or combining data streams, and multiplexing ordemultiplexing data on carriers with different wavelengths.

FIG. 1B is a diagram illustrating an exemplary photonically-enabledintegrated circuit, in accordance with an example embodiment of thedisclosure. Referring to FIG. 1B, there is shown thephotonically-enabled integrated circuit 130 comprising electronicdevices/circuits 131, optical and optoelectronic devices 133, a lightsource interface 135, a chip front surface 137, an optical fiberinterface 139, CMOS guard ring 141, and a surface-illuminated monitorphotodiode 143.

The light source interface 135 and the optical fiber interface 139comprise grating couplers, for example, that enable coupling of lightsignals via the CMOS chip surface 137, as opposed to the edges of thechip as with conventional edge-emitting/receiving devices. Couplinglight signals via the chip surface 137 enables the use of the CMOS guardring 141 which protects the chip mechanically and prevents the entry ofcontaminants via the chip edge.

The electronic devices/circuits 131 comprise circuitry such as theamplifiers 107A-107D and the analog and digital control circuits 109described with respect to FIG. 1A, for example. The optical andoptoelectronic devices 133 comprise devices such as the couplers103A-103K, optical terminations 115A-115D, grating couplers 117A-117H,optical modulators 105A-105D, high-speed heterojunction photodiodes111A-111D, and monitor photodiodes 113A-113I.

In an example scenario, the high-speed heterojunction photodiodes111A-111D comprise double heterostructure (DH) devices in which thecurrent flowing into the germanium device is injected/extracted throughhighly doped p- and n-type Si/Ge heterojunctions, where the doping issolely in the silicon layers.

FIG. 1C is a diagram illustrating a photonically-enabled integratedcircuit coupled to an optical fiber cable, in accordance with an exampleembodiment of the disclosure. Referring to FIG. 1C, there is shown thephotonically-enabled integrated circuit 130 comprising the chip surface137, and the CMOS guard ring 141. There is also shown a fiber-to-chipcoupler 145, an optical fiber cable 149, and an optical source assembly147.

The photonically-enabled integrated circuit 130 comprises the electronicdevices/circuits 131, the optical and optoelectronic devices 133, thelight source interface 135, the chip surface 137, and the CMOS guardring 141 may be as described with respect to FIG. 1B, for example.

In an example embodiment, the optical fiber cable may be affixed, viaepoxy for example, to the CMOS chip surface 137. The fiber chip coupler145 enables the physical coupling of the optical fiber cable 149 to thephotonically-enabled integrated circuit 130.

FIG. 2 illustrates a germanium photodetector with contacts on germaniumlayers, in accordance with an example embodiment of the disclosure.Referring to FIG. 2, there is shown a photodetector 200 comprising asilicon dioxide (SiO₂) layer 201, a silicon layer 203, SiO₂ layer 205, agermanium layer 207, n-doped germanium layer 209, p-doped germaniumlayer 211, waveguide layers 213, a passivating SiO₂ layer 215, and plugs217A and 217B.

In an example scenario, the photodetector 200 may be formed on asilicon-on-insulator (SOI) wafer, where a silicon layer, e.g., thesilicon layer 203, is on an oxide layer, the SiO₂ layer 203. Inaddition, trenches formed in the silicon layer 203 may be filled withthe SiO₂ layers 205 for electrical and/or optical isolation.

A typical Ge-based integrated photodiode is based on p-n or p-i-njunctions formed in the device body. The photodetector 200 comprises ap-i-n structure formed by the p-doped germanium layer 211, the germaniumlayer 207, and the n-doped germanium layer 209.

Metal contacts, with nearly ohmic transfer functions are normally madein the highly doped p− and n− regions to apply the required bias and toinject and extract the resulting currents. Accordingly, the plugs 217Aand 217B comprise metal contacts formed on the n-doped germanium layer209 and the p-doped layer 211, which are both highly doped to providegood electrical contact but also then causes scattering loss for theoptical mode.

The device built-in potential, which plays a key role in determining therectifying and photocurrent collection functionalities, is normallyestablished by the presence of the doped regions. In these standarddevices, the presence of a Si layer has limited impact on the electricalcharacteristics of the device, and acts as the substrate used to growthe Ge film and to establish the optical connectivity of the device.

FIG. 3A illustrates a germanium photodetector without contacts ongermanium, in accordance with an example embodiment of the disclosure.Referring to FIG. 3A, there is shown a photodiode 300 comprising asilicon layer 301, an n+ silicon layer 303, a p+ silicon layer 305, agermanium layer 307, and contacts 309A and 309B. The n+ and p+ indicatesthat these layers are highly doped, on the order of 10¹⁹ cm⁻³ forsilicon.

In an example scenario, the n+ and p+ silicon layers 303 and 305abutting/surrounding the germanium layer 307 assume an active electricalrole by providing the means through which voltage and currents areapplied and flow as well as the chemical potential difference causingthe rise of the junction built-in voltage. This is accomplished byrealizing a double heterostructure (DH) device in which the currentflowing into the Ge device is injected/extracted through highly doped p-and n-type Si/Ge heterojunctions, where the doping is solely in the n+silicon layer 303 and the p+ silicon layer 305.

The high doping, aided by defects at the Si/Ge interface, helps carriertunneling through the heterojunctions thus realizing a low-drop,quasi-ohmic contact. The opposite doping of the two junctions createsthe built-in potential of the p-i-n structure. The current flow out ofthe double heterostructure device and into an external circuit can nowbe accomplished by standard metal plugs placed on the highly doped(silicided) Si regions, far from the optical mode present in thegermanium layer 307.

In order to create the tunneling junctions, a doping in the silicon onthe order of 10¹⁹ cm⁻³ or more is utilized. The germanium side of theheterojunction can be left undoped since the large density of defectsinduced by the presence of misfit dislocations at the Ge/Simetallurgical interface pins the Fermi level close to the valence bandand provides enough charge to screen potential over a very smalldistance. A high density of states at the hetero-interface can also helptunneling by providing a conduction path for carrier hopping. Theband-alignment is shown in FIG. 3B.

FIG. 3B illustrates the band diagram for the photodetector structureshown in FIG. 3A. Referring to FIG. 3B, there is shown a band diagram320 for a double heterojunction silicon/germanium/silicon p-i-nstructure. As shown, the Fermi level in the germanium layer is pinned tojust above the valence band due to defects at the heterojunctioninterface, although this level may change with a change in the defectdensity. The heterointerfaces and associated defect density may providea quasi-tunnel junction for carriers to and from the germanium layer.

Various double heterostructure designs may be utilized with silicon usedfor the contact structures. For example, vertical or horizontal doubleheterostructures may be utilized as shown in FIGS. 4 and 5.

FIG. 4 illustrates a cross-section of a horizontal doubleheterostructure, in accordance with an example embodiment of thedisclosure. Referring to FIG. 4, there is shown a photodetector 400comprising a SiO₂ layer 401, n+ silicon layer 403, intrinsic siliconlayer 405, p+ silicon layer 407, trenches 409, germanium layer 411, p+germanium layer 412, waveguide layers 413, passivation layer 415, andmetal contacts 417A and 417B.

The waveguide layers 413 may comprise a stack of semiconductor anddielectric layers for optical and electrical confinement of thephotodetector 400 and may be utilized to guide optical waves into thephotodetector 400. The passivation layer 415 may comprise a dielectricmaterial, such as SiO₂ and may provide electrical isolation as well asprotect underlying structures from oxidation, for example.

The contacting scheme of the photodetector 400 is based on a standardPIN homo-junction geometry with the field profile in the photo-sensitiveGe layer being horizontal, but with no contacts required in thegermanium layer 411, which is the absorbing layer. As shown, thephotodetector 400 may comprise a p-i-n structure formed by the p+silicon layer 407, the (largely) undoped germanium layer 411, and the n+silicon layer 403, but with an added p+ germanium layer 412.

To increase the bandwidth of the device, selective doping can beintroduced in the germanium layer 411, e.g., the p+ germanium layer 412,to efficiently redistribute the field in order to facilitate fasterphoto-carrier collection. As an example, in the photodetector 400, p+implants overlapping with the p+-Si layer 407 may be used in thegermanium layer 411, which increases the field in the rest of thephoto-sensitive region and improves the bandwidth without hamperingresponsivity.

It should be noted that the lack of contacts on the germanium enablessuch implant optimization since it decouples the location of the dopingin the germanium from the contacts, thereby significantly reducing therestrictions on where implants can be placed. The optical trenches 409on the side of the germanium layer 411 confine the optical mode and maybe utilized in most waveguide detector designs.

FIG. 5 illustrates a vertical double heterojunction germaniumphotodetector, in accordance with an example embodiment of thedisclosure. Referring to FIG. 5, there is shown a photodetector 500comprising SiO₂ layer 501, n+ silicon layer 503, n-silicon layer 505, p+silicon layer 507, trenches 509, germanium layer 511, waveguide layers513, passivation layer 515, and metal contacts 517A and 517B.

In an example scenario, a vertical double heterostructure device, whichutilizes a vertical hetero-structure geometry, but with the doubleheterojunction contacting technique is shown in FIG. 5. As shown, theprimary junction utilized for the photo-collection process may be then-Si/Ge vertical heterojunction. This configuration is advantageous dueto the specific band alignment of the Ge/Si interface. And, as with thehorizontal double heterostructure device in FIG. 4, the efficiency ofthe detector can also be improved by doping the top part of thegermanium layer 511 p-type. But instead of the approach of contactingthe p-Ge directly, the contact to the anode may be made via a narrowsegment of the p+ silicon layer 507 overlapping with the germanium layer511. Also, configuring a moderate level of doping in the n-silicon layer505 directly underneath the absorbing germanium layer 511, may ensurethat the n-Si/p+Si junction doesn't breakdown at low voltages, whilestill creating enough depletion in the germanium layer 511 to facilitateeffective photo-carrier collection without excess dark current.

FIG. 6 illustrates a surface-illuminated horizontal doubleheterostructure germanium photodetector, in accordance with an exampleembodiment of the disclosure. Referring to FIG. 6, there is shown aphotodetector 600 comprising an intrinsic silicon layer 601, n+ siliconlayer 603, p+ silicon layer 605, germanium layer 607, contacts 609, n+silicon fingers 611, and p+ silicon fingers 613.

In addition to the waveguide detectors shown previously, the doubleheterostructure architecture may also be applied to surface Illuminatedphotodetectors. FIG. 6 shows the implementation of a horizontal junctionsurface-illuminated photodiode. In place of metal fingers and contactson the germanium layer 607, both the anode and cathode contacts may beestablished through fingers 611 and 613 of highly doped silicon thatcreate contact-heterojunctions with the germanium layer 607.

The intrinsic silicon layer 601 may comprise silicon without intentionaldopants and the n+ and p+ silicon layers 603 and 605 may be highly dopedsilicon layers and may extend over the central light sensing area of thephotodetector 600 in the n+ silicon fingers 611 and p+ silicon fingers613.

The contacts 609 may comprise metal layers on the n+ and p+ siliconlayers 603 and 605 and may provide electrical interconnection betweenthe photodetector 600 and other devices. The heterojunctions in thephotodetector 600 are formed by the highly doped silicon fingers 611 and613 on the germanium layer 607, which is the light absorbing layer, sothe metal contacts are spatially separated from the light absorbingregion, thereby reducing scattering.

FIG. 7 illustrates a vertical junction surface-illuminated photodiode,in accordance with an example embodiment of the disclosure. Referring toFIG. 7, there is shown a photodetector 700 comprising n-silicon layer701, n+ silicon layer 703, p+ silicon layer 705, p-germanium layer 707,and metal anode and cathode contacts 709A and 709B, respectively.

In the example shown, the anode contact, i.e., via the p-germanium layer707, may be created through a narrow overlap between the p-germaniumlayer 707 and the p+ silicon 705, which creates the contactingheterostructure. As with the vertical double heterostructure waveguidedetector, moderately doped n-type silicon 701 underneath the germanium707 creates a vertical junction that accomplishes photo-carriercollection. The cathode contact 709B may be created directly on the n+silicon 703. In both the above designs, metal contacts are successfullyremoved from the germanium, while providing sufficient electricalconnectivity to the germanium layer 707 so as not to hamper the quantumefficiency of the device.

FIG. 8 illustrates a test structure for assessing n-type tunnelingcontacts, in accordance with an example embodiment of the disclosure.Referring to FIG. 8, there is shown test structure 800 comprising ap-type silicon layer 801, n+ silicon layers 803A and 803B, undopedgermanium layer 807, and metal contacts 809A and 809B.

In order to control and verify the quality of the n-type and p-typetunneling contacts independently, appropriate test structures may beformed in wafers. The test structure 800 shown in FIG. 8 comprises twon+ layers surrounding the undoped germanium allowing for an assessmentof the n-type silicon/germanium heterojunction tunneling contacts.Current-voltage and capacitance measurements of structures withdifferent doping levels may be utilized to determine proper dopinglevels for desired photodetector operation. The structures may be testedas single structures, or in chains for defect detection.

FIG. 9 illustrates a test structure for assessing p-type tunnelingcontacts, in accordance with an example embodiment of the disclosure.Referring to FIG. 9, there is shown a test structure 900 comprising an-type silicon layer 801, p+ silicon layers 903A and 903B, undopedgermanium layer 907, and metal contacts 909A and 909B. As with the teststructure 800, the test structure 900 comprises the same doping on eachside of the germanium layer 907, but with p-type doping in this case.

Current-voltage and capacitance measurements of structures withdifferent doping levels may be utilized to determine proper dopinglevels for desired photodetector operation. The structures may be testedas single structures, or in chains for defect detection.

FIG. 10 illustrates 4-probe heterostructure test structures, inaccordance with an example embodiment of the disclosure. Referring toFIG. 10, there is shown a Kelvin test structure 1000 comprising n-typesilicon 1001, p+ silicon layer 1003, germanium layer 1007, and probecontacts 1009.

In an example In an example scenario, one or more of the probe contacts1009 may act as force contacts and one or more other probe contacts mayact as sense contacts in a Kelvin probe test. Kelvin (4-probe)structures may be used to measure the actual contact resistance clean ofparasitics due to bulk semiconductor resistance and metal plugsresistance. The same structure with opposite doping can be used to testn-type heterojunctions.

In an example embodiment, a method and system are disclosed forgermanium-on-silicon photodetectors without germanium layer contacts. Inthis regard, aspects of the disclosure may comprise a semiconductor diehaving a photodetector, where the photodetector comprises an n-typesilicon layer, a germanium layer, a p-type silicon layer, and a metalcontact on each of the n-type silicon layer and the p-type siliconlayer, and where the photodetector is operable to: receive an opticalsignal, absorb the optical signal in the germanium layer, generate anelectrical signal from the absorbed optical signal, and communicate theelectrical signal out of the photodetector via the n-type silicon layerand the p-type silicon layer.

The photodetector may comprise a horizontal junction doubleheterostructure where the germanium layer is above the n-type siliconlayer and the p-type silicon layer. An intrinsically-doped silicon layermay be below the germanium layer between the n-type silicon layer andthe p-type silicon layer. A portion of the germanium layer nearest thep-doped silicon layer may be p-doped. The photodetector may comprise avertical junction double heterostructure where the germanium layer isabove a lower-doped n-type silicon layer.

The n-type silicon layer and the p-type silicon layers may be onopposite sides of the lower-doped silicon layer below the germaniumlayer where the p-type silicon layer and the lower-doped n-type siliconlayer are in contact with the germanium layer while the n-type siliconlayer is not. A top portion of the germanium layer may be doped p-type.The photodetector may comprise a surface-illuminated doubleheterostructure photodetector. The n-type silicon layer and the p-typesilicon layer in the surface-illuminated double heterostructurephotodetector may comprise interdigitated fingers.

In another example scenario, a method and system are disclosed forgermanium-on-silicon photodetectors without germanium layer contacts. Inthis regard, aspects of the disclosure may comprise a semiconductor diehaving a double heterostructure photodetector, where the doubleheterostructure photodetector comprises an n-type silicon layer, agermanium layer with p-type doping in a portion of the germanium layer,a p-type silicon layer, and a metal contact on each of the n-typesilicon layer and the p-type silicon layer, and where the doubleheterostructure photodetector is operable to: receive an optical signal,absorb the optical signal in the germanium layer, generate an electricalsignal from the absorbed optical signal, and communicate the electricalsignal out of the photodetector via the n-type silicon layer and thep-type silicon layer.

As utilized herein the terms “circuits” and “circuitry” refer tophysical electronic components (i.e. hardware) and any software and/orfirmware (“code”) which may configure the hardware, be executed by thehardware, and or otherwise be associated with the hardware. As usedherein, for example, a particular processor and memory may comprise afirst “circuit” when executing a first one or more lines of code and maycomprise a second “circuit” when executing a second one or more lines ofcode. As utilized herein, “and/or” means any one or more of the items inthe list joined by “and/or”. As an example, “x and/or y” means anyelement of the three-element set {(x), (y), (x, y)}. In other words, “xand/or y” means “one or both of x and y”. As another example, “x, y,and/or z” means any element of the seven-element set {(x), (y), (z), (x,y), (x, z), (y, z), (x, y, z)}. In other words, “x, y and/or z” means“one or more of x, y and z”. As utilized herein, the term “exemplary”means serving as a non-limiting example, instance, or illustration. Asutilized herein, the terms “e.g.,” and “for example” set off lists ofone or more non-limiting examples, instances, or illustrations. Asutilized herein, circuitry or a device is “operable” to perform afunction whenever the circuitry or device comprises the necessaryhardware and code (if any is necessary) to perform the function,regardless of whether performance of the function is disabled or notenabled (e.g., by a user-configurable setting, factory trim, etc.).

While the disclosure has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present disclosure. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present disclosure without departing from itsscope. Therefore, it is intended that the present disclosure not belimited to the particular embodiments disclosed, but that the presentdisclosure will include all embodiments falling within the scope of theappended claims.

What is claimed is:
 1. A method for optical communication, the methodcomprising: in a semiconductor die having a photodetector, thephotodetector comprising an n-type silicon layer, a germanium layer, ap-type silicon layer, and a metal contact on each of the n-type siliconlayer and the p-type silicon layer, wherein a top surface of thegermanium layer is above top surfaces of both the n-type silicon layerand the p-type silicon layer and a bottom surface of the germanium layeris below the top surfaces of both the n-type silicon layer and thep-type silicon layer, and wherein contacts to the photodetector are onopposite sides of the germanium layer with no contacts on a top surfaceof the germanium layer: receiving an optical signal; absorbing theoptical signal in the germanium layer; and generating an electricalsignal from the absorbed optical signal.
 2. The method according toclaim 1, wherein the photodetector comprises a horizontal junctiondouble heterostructure.
 3. The method according to claim 1, wherein anintrinsically-doped silicon layer is below the germanium layer betweenthe n-type silicon layer and the p-type silicon layer.
 4. The methodaccording to claim 1, wherein a portion of the germanium layer nearestthe p-doped silicon layer is p-doped.
 5. The method according to claim1, wherein the photodetector comprises a vertical junction doubleheterostructure where the germanium layer is above a lower-doped n-typesilicon layer.
 6. The method according to claim 5, wherein the n-typesilicon layer and the p-type silicon layers are on opposite sides of thelower-doped silicon layer below the germanium layer where the p-typesilicon layer and the lower-doped n-type silicon layer are in contactwith the germanium layer while the n-type silicon layer is not.
 7. Themethod according to claim 5, wherein a top portion of the germaniumlayer is doped p-type.
 8. The method according to claim 1, wherein thephotodetector comprises a surface-illuminated double heterostructurephotodetector.
 9. The method according to claim 8, wherein the n-typesilicon layer and the p-type silicon layer in the surface-illuminateddouble heterostructure photodetector comprise interdigitated fingers.10. The method according to claim 8, wherein the n-type silicon layerand the p-type silicon layer in the surface-illuminated doubleheterostructure photodetector comprise ring-shaped structures at anouter edge of the surface-illuminated double heterostructurephotodetector.
 11. A system for communication, the system comprising: asemiconductor die having a photodetector, the photodetector comprisingan n-type silicon layer, a germanium layer, a p-type silicon layer, anda metal contact on each of the n-type silicon layer and the p-typesilicon layer, wherein a top surface of the germanium layer is above topsurfaces of both the n-type silicon layer and the p-type silicon layerand a bottom surface of the germanium layer is below the top surfaces ofboth the n-type silicon layer and the p-type silicon layer, and whereincontacts to the photodetector are on opposite sides of the germaniumlayer with no contacts on a top surface of the germanium layer, thephotodetector being operable to: receive an optical signal; absorb theoptical signal in the germanium layer; and generate an electrical signalfrom the absorbed optical signal.
 12. The system according to claim 11,wherein the photodetector comprises a horizontal junction doubleheterostructure.
 13. The system according to claim 11, wherein anintrinsically-doped silicon layer is below the germanium layer betweenthe n-type silicon layer and the p-type silicon layer.
 14. The systemaccording to claim 11, wherein a portion of the germanium layer nearestthe p-doped silicon layer is p-doped.
 15. The system according to claim11, wherein the photodetector comprises a vertical junction doubleheterostructure where the germanium layer is above a lower-doped n-typesilicon layer.
 16. The system according to claim 15, wherein the n-typesilicon layer and the p-type silicon layers are on opposite sides of thelower-doped silicon layer below the germanium layer where the p-typesilicon layer and the lower-doped n-type silicon layer are in contactwith the germanium layer while the n-type silicon layer is not.
 17. Thesystem according to claim 15, wherein a top portion of the germaniumlayer is doped p-type.
 18. The system according to claim 11, wherein thephotodetector comprises a surface-illuminated double heterostructurephotodetector with the n-type silicon layer and the p-type silicon layerin the surface-illuminated double heterostructure photodetectorcomprising interdigitated fingers.
 19. The system according to claim 18,wherein the photodetector comprises a surface-illuminated doubleheterostructure photodetector with the n-type silicon layer and thep-type silicon layer in the surface-illuminated double heterostructurephotodetector comprising ring-shaped structures at an outer edge of thesurface-illuminated double heterostructure photodetector.
 20. A systemfor communication, the system comprising: a semiconductor die having adouble heterostructure photodetector, the double heterostructurephotodetector comprising an n-type silicon layer, a germanium layer withp-type doping in a portion of the germanium layer, a p-type siliconlayer, and a metal contact on each of the n-type silicon layer and thep-type silicon layer, wherein a top surface of the germanium layer isabove top surfaces of both the n-type silicon layer and the p-typesilicon layer and a bottom surface of the germanium layer is below thetop surfaces of both the n-type silicon layer and the p-type siliconlayer, and wherein contacts to the photodetector are on opposite sidesof the germanium layer with no contacts on a top surface of thegermanium layer, the double heterostructure photodetector being operableto: receive an optical signal; absorb the optical signal in thegermanium layer; and generate an electrical signal from the absorbedoptical signal.